Described herein is an apparatus for and a method of measuring the operational effectiveness of an electrophoretic ink display device, and a method of making display devices using such method. More particularly, described is an apparatus for and a method of measuring the charges of the particles of an electrophoretic display medium of an electrophoretic display device to determine if the device is able to properly display images, and to optimize performance parameters for preparation of additional similar display devices.
An advantage associated with the apparatus and methods described herein is that a given electrophoretic ink and/or display device can be readily evaluated for effectiveness in displaying images, thereby improving quality control in the manufacture of display devices. Using the evaluation method herein, additional display devices having a same high quality display can be replicated.
Electrophoretic displays are known in the art. Known electrophoretic display devices include a device in which charged pigment particles are dispersed along with conductive additives such as charge control agents in a clear or colored insulating liquid of matching specific gravity, the dispersion being encapsulated and the encapsulated spheres being contained between two parallel conducting electrode panels. Such a display is available from E-Ink. Examples of charge control agents used have included bis-(2-ethyl hexyl)sodium sulfosuccinate and basic barium petronate (BBP). Dissociation of the charge control agent into positive and negative ionic species in the dielectric fluid results in preferential surface absorption of ions of one polarity by the particles. The particles therefore become charged. The resulting dispersion contains a complex mixture of particles including charged particles, excess free ions and counter-ions. Due to the presence of excess free ions, such electrophoretic display inks are characterized by high electrical conductivity. Conductivity has been shown to increase with concentration of the added charge control agent, and is typically 100-1000 times higher compared to the dielectric fluid. High conductivity of the ink results in increased power consumption and slower switching speed of the display.
Another known type of electrophoretic display device is comprised of charged pigment particles in a sealed (air-filled) device. Such a display is available from Fuji Xerox.
In addition, U.S. patent applications Ser. Nos. 11/419,472 and 11/419,436, each incorporated herein by reference in its entirety, describe a novel electrophoretic display device comprised of a multiplicity of individual reservoirs containing a display fluid between conductive substrates, at least one of which is transparent, wherein the display fluid comprises pigment particles in a liquid medium and wherein the display fluid has an electrical conductivity of about 10−11 to about 10−15 S/m. The display medium is desirably substantially free of charge control additives and ionic species. A significant difference between the inks for these display devices and conventional electrophoretic inks that include conductive additives is the low electrical conductivity of the novel inks due to the omission of charge directors and charge control agents from the fluid medium.
U.S. Pat. No. 4,272,596, incorporated herein by reference in its entirety, illustrates an electrophoretic display structure. The electrophoretic display device comprises a white marking material such as titanium dioxide suspended in a colloidal dispersion containing black colorant such as colloidal size iron oxide particles known as ferrofluids. Image formation is achieved electrophoretically by selective application of an electric field across the imaging suspension. In particular, a pair of electrodes associated so as to form a cavity therebetween, which cavity is filled with the aforementioned suspension medium. A source of electrical potential is coupled to the electrodes and when an electric field is applied, the marking particles form an image as they follow the field.
U.S. Pat. No. 6,113,810, incorporated herein by reference in its entirety, describes a dielectric dispersion for use in an electrophoretic display that includes a dielectric fluid, a first plurality of particles of a first color having a surface charge of a selected polarity dispersed within the dielectric fluid and a second plurality of particles of a second color having a surface charge of opposite polarity to that of the first plurality and a steric repulsion thereto preventing coagulation of the first and second plurality of particles. Each set of particles is formed with unique secondary and functional monomers. Corresponding charge control agents are added to the dispersion to establish opposite polarities on the respective particles.
U.S. Pat. No. 6,017,584, incorporated herein by reference in its entirety, discloses electrophoretic displays and materials useful in fabricating such displays. In particular, encapsulated displays are disclosed in which particles encapsulated therein are dispersed within a suspending, or electrophoretic, fluid. This fluid may be a mixture of two or more fluids or may be a single fluid. The displays may further comprise particles dispersed in a suspending fluid, wherein the particles contain a liquid. In either case, the suspending fluid may have a density or refractive index substantially matched to that of the particles dispersed therein. Application of electric fields to the electrophoretic displays affects an optical property of the display.
U.S. Pat. No. 6,577,433, incorporated herein by reference in its entirety, discloses an electrophoretic display liquid composition for use in an electrophoretic display device that has a multiplicity of individual reservoirs, each containing the display liquid of two sets of particles dispersed in a transparent liquid system as well as at least one charge director dissolved or dispersed in the liquid system, or physically embedded on the surface of the particles or chemically bonded on the surface of the surface of the particles, the two sets of particles exhibiting different, contrasting color and different charging properties from each other. The charge director(s) may include a metal salicylate compound. The particles may be modified with charge controlling agents, and may also include a set of magnetic particles. The transparent liquid system may include two immiscible liquids having different densities with the sets of particles having densities in between the densities of the two immiscible liquids such that the particles rest at an interface between the two immiscible liquids.
U.S. Pat. No. 6,525,866, incorporated herein by reference in its entirety, discloses an electrophoretic display liquid composition for use in an electrophoretic display device that has a multiplicity of individual reservoirs, each containing the display liquid of at least two sets of particles dispersed in a transparent liquid system, the at least two sets of particles exhibiting different, contrasting color and different charging properties from each other, and at least one of the sets of particles containing flow aid particles as additives upon an external surface of the particles. Preferred flow aid additives include silica and titania particles.
Electrophoretic display is thus based on the migration of charged particles under the influence of an electric field.
Characterization methods for traditional electrophoretic inks include: Q/M (commonly used in liquid toner technology), optical density measurements, and mobility measurements. Due to the complex nature of the composition of conventional electrophoretic inks containing conductive additives, the actual charged species can consist of partially adsorbed charge additives, charge control agents, excess free ions and counter-ions, along with the particles. Thus, for any system that relies on conductive additives for particle charging, there is no technique for measuring the charge on the particles alone. Moreover, no such technique is known for any other type of electrophoretic display. This makes it difficult (1) to optimize the particle making process parameters and produce toners with reproducible performance characteristics, and (2) to develop materials packages with optimal performance.
What is desired is an apparatus for and a method of measuring the parameters of an electrophoretic ink representative of operational characteristics of the ink in a display device, and/or for measuring operational effectiveness of an electrophoretic ink display, in order to confirm that the display is capable of adequately and properly displaying images, and a method of making display devices having high quality display capabilities.